In a turbocharger which compresses intake air for an engine, a bypass path is provided to prevent over-rotation of the turbocharger due to increased pressure of the exhaust gas. The bypass pass bypasses the turbine from the exhaust gas inlet side of the outlet side of the turbine of the turbocharger and is opened and closed by the wastegate valve.
FIG. 9 shows a structure of a supply and exhaust device of the engine to which a wastegate valve is applied.
In FIG. 9, a supply path 58 is connected to an engine 52 via a supply manifold 56 and an exhaust path 60 is connected to the engine 52 via an exhaust manifold 54.
In the supply path 58, a compressor 70a of the turbocharger 70 is provided. The compressor 70a is coaxially arranged with and driven by a turbine 70b. On a downstream side of the compressor 70a in the supply path 58, an intercooler 62 is provided to cool the intake air flowing in the supply path 58 by heat exchange with ambient air. On a downstream side of the intercooler 62 in the supply path 58, a throttle valve 64 is provided to regulate a flow rate of the intake air flowing in the supply path 58.
In the exhaust path 60, the turbine 70b of the turbocharger 70 is provided. The turbine 70b is driven by the exhaust gas from the engine 52. In the exhaust path 60, a bypass path 2 connects an exhaust gas inlet side of the turbine 70b to an exhaust gas outlet side to bypass the turbine 70b. In the bypass path 2, a wastegate valve 4 is provided to open and close the bypass path 2.
With the above structure, once the engine 52 starts, the exhaust gas from the engine 52 is collected into an exhaust gas manifold 54 and fed to the turbine 70b of the turbocharger 70 through the air exhaust path 60 to drive the turbine 70b. 
In the turbocharger 70, the compressor 70a is driven by the turbine 70b to compress the intake air. The compressed intake air is supplied through a supply manifold 56 to the engine 2 via the supply path 58, the intercooler 62 and the supply path 58.
In the case of preventing the over-rotation of the turbocharger 2, the exhaust gas from the engine 2 may be led to the bypass path to bypass the turbine 70b by opening the wastegate valve 4.
For instance, a pressure at an outlet of the compressor 70a may be detected and once the detected pressure exceeds a preset threshold value, the wastegate valve 4 may be opened to allow the exhaust gas to bypass the turbine 70b, thereby preventing the over-rotation of the turbocharger.
As a conventional wastegate valve, a swing-type valve having a valve seat in a plane perpendicular to an axial direction of the bypass path at an open end thereof. This type of the wastegate valve is disclosed in Patent Literature 1.
FIG. 11 is a side view of the conventional wastegate valve. FIG. 12 is a fragmentary side view taken in a direction of an arrow B of FIG. 11. FIG. 11 and FIG. 12 show local sectional views.
In FIG. 11 and FIG. 12, a wastegate valve 104 includes a valving element 144 and a valve seat 142 on which the valving element 144 is placed. The valving element 144 is supported by a pivot shaft 146 via a support arm 148 pivotable in a direction of an arrow W′ around an axial center r′ of the pivot shaft. The wastegate valve 104 is closed by placing the valving element 144 on the valve seat 142.
In FIG. 11 and FIG. 12, the valve seat 142 is arranged in the plane which is perpendicular to the axial direction of the bypass path.
To open the valving element 144, the pivot shaft 146 is turned so that the valving element 144 is moved away from the valve seat 142 in the direction of the arrow W′ to a valve-open position.